1. Field of the Invention
The present invention relates to a vibration wave driven motor adoptable for video apparatus, printer, semiconductor device or the like. More specifically, the present invention is directed to vibration absorbing means in a travelling wave type of vibration wave driven motor.
2. Related Background Art
A vibration wave driven motor or actuator is known in the art in which a travelling vibration wave is generated on an elastic member and a movable member such as a rotor is moved by the vibration wave. This type of motor has advantages that it is small in size and- that a high torque can be obtained even at a low speed. For these advantages, this type of motor has recently been employed in a single-lens reflex camera as a motor for driving the photographing lens system.
FIG. 3 is a longitudinal sectional view of a prior art photographing lens system in a single-lens reflex camera having a vibration wave driven motor for driving the lens system.
Referring to the figure, the optical axis of the lens system is denoted by L. The reference numeral 1 denotes a metallic elastic member in a form of ring the rotational center of which corresponds with the optical axis L. The elastic member 1 has a piezo-electric element 2 fixed to the lower portion of the member by means of adhesives. The piezo-electric element 2 serves as an electromechanical energy conversion element and may be formed of, for example, PZT. In a method known per se, ultrasonic driving signals having different phases are applied to the piezo-electric element 2. In response to the applied signal, the elastic member 1 vibrates and there is generated a travelling vibration wave which travels round along the circumference of the elastic member 1 constituting a vibration element. The reference numeral 3 denotes a ring rotor as a movable member. An end of the rotor 3 is pressure-contacted with the upper surface of the elastic member 1. At the other end, the rotor 3 has a first vibration absorber 5 which may be made of rubber or the like. The reference numeral- 4 denotes a vibration isolator formed of felt or the like. The vibration isolator 4 is pressed against the piezo-electric element 2 by two plate springs 9 through a felt base 8.
FIG. 4 is an enlarged view of the essential part of the photographing lens system of the single-lens reflex camera shown in FIG. 3.
At the above-mentioned first vibration absorber 5, the rotor 3 is in close contact with a coupling ring plate 22. As seen best from an exploded perspective view of FIG. 5, the coupling ring plate 22 is fixedly coupled to an output transmission member 25 by six fastening screws 24 through clearance holds 22a and threaded holes 25a. The output transmission member 25 rotates about the optical axis L. The output transmission member 25, a ball 10 and ball 11 races 13, 14 together constitute a ball bearing. The ball races 13 and 14 are fixed to an outer tube 12 of the lens system. The outer tube 12 is coupled to a stationary tube 11 and they are fixed on a camera mount 19. At the forward end, the output transmission member 25 has a coupling roller 15 which is engaged in a key way (not shown) formed on a movable ring 17 in the direction along the optical axis. The movable ring 17 holds a focusing lens 27. The movable ring 17 has a thread portion 17a in helicoidal connection with a thread portion 18a of the stationary inner tube 18.
With the arrangement described above, when the output transmission member 25 is rotated, the movable ring 17 can be moved in the direction along the optical axis while rotating through the coupling roller 15 at the same time.
In this apparatus, therefore, the focusing lens 27 can be moved along the optical axis for focusing by rotating the rotor 3 by a travelling wave generated on the vibration member in response to an AF signal from the camera side or a driving signal from a manual ring 16.
The prior art apparatus as described above has, however, some drawbacks caused by the propagation of undesirable vibration wave. The coupling ring plate 22 in the prior art structure is not a perfect rigid body and, therefore, it is impossible to completely attenuate unnecessary and undesirable vibration of the rotor 3 by the first vibration absorber 5. Rather, such unnecessary vibration is transmitted to the coupling plate 22 and also to the output transmission member 25. The transmitted unnecessary vibration is reflected by the end of the output transmission member. The reflected vibration goes back to the rotor 3 through the coupling plate 22 and the first vibration absorber 5. This unnecessary vibration has an adverse effect on the contact between the rotor and the vibration element. As a result, there is generated noise. Further, the lowering of output is often caused by it.